1. Field of the Invention
The present invention relates to an adsorption process for treatment of a gas mixture comprising at least one main constituent to be produced and impurities to be separated from said mixture, especially for the production of carbon monoxide with streams having predetermined hydrogen/carbon monoxide ratios.
2. Related Art
Throughout the text, the gas pressures indicated are in bar absolute.
Such a treatment process is widely used to separate “noble” constituents to be produced, that are contained in the gas mixture, from undesirable constituents, generally denoted by the term “impurities”.
The typical process is cyclic and involves at least two adsorbers, of at least two adsorption units having respectively several adsorbers operating in common, which follow in an offset manner the same operating cycle. This cycle conventionally comprises an adsorption phase, during which the corresponding adsorber is subjected to the gas mixture and adsorbs the impurities thereof, and a regeneration phase, during which the adsorber is subjected to a regeneration gas and is desorbed of the impurities that it had previously adsorbed.
Depending on whether or not the regeneration phase is accompanied by heating of the regeneration gas, it is common practice to distinguish cycles called TSA (Temperature Swing Adsorption) cycles from cycles called PSA (Pressure Swing Adsorption) cycles.
It is also known that the adsorbers may be subjected to depressurization and repressurization steps and to a step of paralleling the adsorbers, during which the total stream of treated gas is obtained both by the treatment of a first flow of gas by at least one adsorber terminating its adsorption phase and by the treatment of a second flow of gas to be treated by at least one other adsorber starting its adsorption phase. This paralleling is conventionally intended to prevent pressure surges in the stream of treated gas during passage in production from one adsorber to another, especially in order to take into account the operating time of the valves that implement the paralleling operation.
However, adsorption treatment cycles have drawbacks during transient periods at the start of the adsorption and regeneration phases, as partly explained in document EP-A-00 748 765.
That document describes a carbon monoxide production plant comprising a cryogenic production unit and, upstream of the latter, a treatment unit that employs a process of the type defined above. This plant is intended to retain the water and carbon dioxide of a gas mixture rich in carbon monoxide and in hydrogen coming from a hydrocarbon steam reforming unit. Fixing the carbon monoxide by the adsorbent of the adsorber that is starting its adsorption phase causes an appreciable reduction in the carbon monoxide content of the stream output by this adsorber, together with fluctuations in the flow rate of this stream. The solution proposed in EP-A-0 748 765 consists in interposing, between the adsorption treatment unit and the cryogenic carbon monoxide production unit, a tank filled with an adsorbent having an affinity for carbon monoxide.
This solution proves to be particularly expensive in terms of investment, is not very modular and attains only to the transient period when each adsorber returns to production, whereas similar transient phenomena occur at the start of the regeneration phase of each adsorber, the stream leaving the adsorbers exhibiting large fluctuations in content and in flow rate.